1. Field of the Invention
The invention relates to a communications system which has at least one slave station controlled by a master station. The slave station(s) here cannot operate without the master station.
A communications system can be realized, for example, by means of the I.sup.2 C (Inter IC) bus. A plurality of master stations and a plurality of slave stations are connected to the I.sup.2 C bus via open-collector outputs. The master stations control the slave stations using addresses which are known to them and which are permanently stored in the slave stations.
It is possible, in the case of the I.sup.2 C bus, for two master stations simultaneously to attempt to access the bus in order to address slave stations. For this case, a so-called arbitration procedure is provided for the I.sup.2 C bus. In this procedure, the competing master stations simultaneously transmit bit by bit in each case one individual identification code, stored in them, into a precharged line of the bus via their open-collector outputs. The potential of the line is drawn to ground as soon as one of the bits which is about to be output has a high level, even if the bit of the other master station has a low level. Each of the master stations monitors whether that bit of its identification code which has just been output has a high level when the line is being discharged. If this is not the case, the master station deactivates itself and that master station whose identification code with all its bits is decisive for the successive potential states of the line manages to access the bus.
A disadvantage of the I.sup.2 C bus is that the maximum data rate is limited by the exclusive use of open-collector outputs.